For homes which are air conditioned, and have ventilated crawl spaces in hot humid climates, is insulating the underside of the floor worthwhile and is a vapor barrier recommended?

First I would recommend grading the soil well so that there is no standing water during rains and from improperly directed gutters or lawn sprinklers. Then put a vapor barrier (6mil poly) on the ground and secure it well and ventilate the crawlspace very well e.g. >25% free vent area by lattice or staggered brickwork. If you do this then you will have eliminated one of the main driving forces, water from the ground.

The next thing you need to worry about is making sure you have absolutely tight air distribution ductwork. We don't want any supply duct leaks which create negative pressures in the house and can suck moisture through the floor and cause mold under kitchen and bath vinyl flooring (which we see in manufactured housing). Also make sure you have adequate return air pathways in the bedroom doors so that you can not create negative pressures by closing off the interior doors.

After doing all this insulate the floor (really for winter time comfort and preventing dust mites in ... more.

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